The overall purpose of this project is to clarify our understanding of myofibrillar assembly and turnover in the heart. Experiments will employ electron microscopic autoradiography and our new methods for the separation and biochemical analysis of newly synthesized myofilament populations from myofibrils. Preliminary studies suggest a rapid internalization of myofilament protein following attachment at the fibrillar periphery. We propose to further define the nascent filament population using isolated rat left atrial tissue. This in vitro system will permit us to pulse label muscle protein and to test, in a controlled manner, the effects of energy, protein synthesis, insulin, branched chain amino acids, microtubules and other factors on myofibrillar assembly and degradation. In addition, patterns of myofibrillar assembly and turnover will be examined during several kinds of cardiac hypertrophy and regression. Finally, we will attempt to demonstrate aspects of myofibrillar assembly and degradation using cell-free preparations. These studies may give insight into the adaptation of the heart to increased or decreased work load and to heart failure which often follows hypertrophy.